The present invention relates to an apparatus for sensing fluid pressure, under dynamic and static fluid flow conditions, wherein the magnitude of the sensed pressure is represented by mechanical displacement of the measuring apparatus.
The principle of operation of Bourdon tube devices is well-known; such devices are typically closed end curved tubes which react to changes of the pressure of the material, either gas or liquid, contained within the tube by increasing or decreasing the tube curvature as a function of these pressures. Since the internal surface area of a curved, preferably flattened, tube is greater on the outer wall curvature than the inner wall curvature, increases in internal pressure cause a greater differential force on the outer wall curvature, thereby tending to straighten the curved tube. These principles have been utilized for many years to provide indicating and sensing devices wherein pressure changes can be related to the phenomena desired to be measured. For example, Bourdon tube devices have been used to measure temperature by enclosing a liquid in a closed circuit including a temperature sensing bulb and a Bourdon tube, wherein increasing liquid temperature develops increasing pressure which in turn causes physical deflection of the Bourdon tube, such as is described in U.S. Pat. No. 2,892,063, issued June 23, 1959.
In a typical operating environment a Bourdon tube device is connected with its opened end coupled to a source of liquid or gas, and its closed end is permitted freedom of movement, for the deflection of the closed end in response to pressure changes is used as the pressure indicating mechanism. Since the deflection of the closed end is gradual and more or less linearly related to pressure changes within the tube, and since the deflection is usually observable over only fairly small ranges of distance, it is frequently desirable to amplify this movement by means of levers attached to the closed free end of the tube. If it is desired to develop an instantaneous indication at a certain pressure or tube deflection position, various forms of electrical and mechanical toggle mechanisms may be utilized in connection with the closed free end of the tube. For example, U.S. Pat. No. 3,188,419 discloses a Bourdon tube pressure switch assembly wherein the closed end of the tube is place adjacent a microswitch actuator such that the inherent snap actuating mechanism of the microswitch becomes activated at a particular deflection angle of the tube end. As another example U.S. Pat. No. 1,838,377, issued Dec. 29, 1931 discloses a combination circuit controller and indicating mechanism wherein a mercury bulb having switch contacts is connected to the Bourdon tube and deflection of the tube causes the mercury bulb to become unbalanced and thereby causes the mercury fluid within the tube to flow to one end or the other completing an electrical circuit.
Many other applications of Bourdon tube devices are known in the art for measuring pressure and pressure-related phenomena and for indicating same and for providing signal generating means at predetermined measured levels. In all of these applications it is conventional to utilize a tube having a closed end which is relatively freely movable so as to generate the necessary pressure response motion. This construction is a disadvantage when it is desirable to measure liquids having contaminants therein, or which are subject to curing or drying inside the tube, thereby changing the physical operating characteristics of the device and damaging its effectiveness. As a result, Bourdon tube devices have not been utilized widely for measuring pressure and related phenomena for liquids such as paint, lacquer, thermo-setting materials, and any other liquid having a tendency to cure or harden or become deposited in solid form under static flow conditions.
In applications requiring the transfer of paint or other similar materials under high pressures it is desirable to provide a pressure sensing and control mechanism which cannot become adversely affected by the material. Leakage and curing of the material frequently damage such mechanisms unless they are properly designed and maintained. U.S. Pat. No. 3,614,352, issued Oct. 19, 1971 and owned by the assignee of the present invention, discloses a pressure response and control assembly suitable for such applications. This patent describes a movable plunger which is in direct contact with a pressurized paint supply, and which passes through a sealed housing to actuate an electrical switch. The switch controls an electric motor which is mechanically coupled to a reciprocating pump for developing paint pressure, and the mechanism may be adjusted to activate and deactivate the pump to develop paint pressures over a specified range. The range of pressures which are typically controlled by devices of this type are 0-3000 pounds per square inch (p.s.i.).
A disadvantage suffered by the foregoing device and other similar devices is in that a movable element of the device is usually in direct contact with the fluid whose pressure is being monitored. Contaminants, curing and leakage problems, and occasionally other factors may degrade or damage the pressure sensing mechanism to cause unreliable or faulty operation. It is therefore desirable to provide a pressure sensing and control device which does not have movable slides, plungers or other similar elements in direct contact with the material being measured.